A conventional tachometer mounted on a vehicle has an imprecise display range because engine r.p.m. varies over a wide range. Further, during idle engine adjustments, it is necessary to accurately set engine r.p.m. at a precise value. However, prior art tachometers have such a poor resolution because of the wide display range that it is difficult to accurately read idling r.p.m.
Moreover, it is difficult for a display device having an analog display, such as an ammeter, to ensure a highly accurate display over all r.p.m. ranges. Therefore, accuracy at low r.p.m. ranges (i.e., idling r.p.m.) is usually sacrificed. Thus, it is difficult to read out idling r.p.m. accurately.
There is also a bar-graph display device which responds to a digital system output in which a number of juxtaposed display segments are sequentially illuminated according to the corresponding r.p.m. If the number of segments were to be increased to enhance resolution, the display control device would become increasingly complicated and even more difficult to read. A further difficulty is that production costs would increase proportionately.